First radio terminal, method to operate a first radio terminal, and method to operate a radio communications network

ABSTRACT

A method to operate a first radio terminal of a radio communications network, is provided, wherein the method comprises: determining a SPS configuration for the first radio terminal, wherein the SPS configuration indicates a set of SPS, semi-persistently scheduled, radio resources of a shared device-to-device radio channel, in particular of a sidelink channel, SL-CH, wherein the indicated SPS radio resources are intended for a transmission by the first radio terminal; transmitting, towards at least one second radio terminal, data via a first subset of the set of the SPS radio resources; selecting at least one sensing resource out of the set of SPS radio resources; sensing the at least one sensing resource; determining a contention indicator in dependence on the sensing of the at least one sensing resource; and refraining from transmitting data via a second subset of the set of SPS radio resources if the contention indicator indicates a contention situation at the at least one sensing resource.

BACKGROUND OF THE INVENTION

The invention concerns a first radio terminal, a method to operate thefirst radio terminal, and a method to operate a radio communicationsnetwork.

Vehicle-to-Everything (V2X) is a system that enables passing informationamong vehicles, between a vehicle and infrastructure, between a vehicleand a network, or between a vehicle and other mobile devices. Thissystem uses a direct communication technology known either as directshort-range communication (DSRC), in IEEE standards, or as sidelinkcommunication, in 3GPP standards. In general, this direct sidelinkcommunication channel is the direct channel between two mobile devicesor a fixed infrastructure node.

SUMMARY OF THE INVENTION

According to a first aspect of the description a first radio terminal ofa radio communications network, in particular a V2X UE of a V2X network,is provided, wherein said first radio terminal comprises at least oneprocessor, at least one memory comprising computer program code, atleast one communication module, and at least one antenna, wherein thecomputer program code and the memory are configured, together with theat least one processor, the at least one communication module, and theat least one antenna, to cause the radio terminal at least to: determinea SPS configuration for the first radio terminal, wherein the SPSconfiguration indicates a set of SPS, semi-persistently scheduled, radioresources of a shared device-to-device radio channel, in particular of asidelink channel, SL-CH, wherein the indicated SPS radio resources areintended for a transmission by the first radio terminal; transmit,towards at least one second radio terminal, data via a first subset ofthe set of the SPS radio resources; select at least one sensing resourceout of the set of SPS radio resources; sense the at least one sensingresource; determine a contention indicator in dependence on the sensingof the at least one sensing resource; and refrain from transmitting datavia a second subset of the set of SPS radio resources if the contentionindicator indicates a contention situation at the at least one sensingresource.

For example, the SPS configuration determination involves that the firstradio terminal determines the SPS configuration via sensing, via controlchannel decoding and reservation decoding, via upper layer messages,e.g., (pre-)configuration, etc.

The proposed scheme provides an adaptive distributed SPS scheme. Due tothe sensing operation at the cost of leaving out transmissionopportunities of the subset of SPS sidelink resources, other radioterminals in the vicinity are able to start or continue their sidelinktransmissions. Both, the centrally scheduling case and distributedscheduling case, benefit.

According to an advantageous example, the first radio terminal isfurther configured to: transmit, towards the at least one second radioterminal, data via the second subset of the set of SPS radio resourcesif the contention indicator indicates no contention situation at the atleast one sensing resource.

Advantageously, a reselection procedure is provided allowing areselection of the SPS resources, i.e., to avoid communicationdisruption or sensing disruption.

According to an advantageous example, the first radio terminal isfurther configured to: determine at least one auxiliary radio resource,wherein the at least one auxiliary radio resource and the set of SPSradio resources are disjoint; transmit, towards the at least one secondradio terminal, data via the at least one auxiliary radio resource.

Using the auxiliary radio resources for transmission of payload datahelps to avoid sequential collision of Long SPS transmissions bydifferent radio terminals. The SPS reservations are protected fromdeterioration. The auxiliary radio resources are determinedout-of-sequence with regard to the SPS radio resources. Therefore,transmission diversity is increased and QoS requirements can bemaintained leaving out or reducing a transmission via the sensingresource.

According to an advantageous example, the data transmitted via the atleast one auxiliary radio resource comprises an out-of-sequenceindicator, which indicates that the auxiliary radio resource is anauxiliary radio resource.

Other radio terminals sensing the out-of-sequence-indicator getknowledge of the function, in particular that the auxiliary radioresource are not SPS resources, and may refrain from transmitting viathe auxiliary radio resources detected.

According to an advantageous example, the selection of the at least onesensing resource comprises to: select the at least one sensing resourceout of the set of SPS resources upon expiry of a time period since thefirst transmission via the set of SPS resources or upon reaching anumber of transmissions conducted via the set of SPS resources.

The time period provides a deterministic behavior of the first radioterminal in using the SPS radio resources and sensing.

According to an advantageous example, the time period or the number oftransmissions is determined in dependence on at least one of thefollowing: a measured channel load of the physical device-to-devicechannel, a present status of an egress queue of the first radioterminal, and a QoS indicator.

An adaptive sensing is provided by considering, for example, the channelload. Therefore, the distributed transmission benefits as the firstradio terminal adapts its behavior to its surrounding radio situation.

According to an advantageous example, determining the contentionindicator comprises: decode SCI, Sidelink Control Information, presentin the received at least one sensing resource; determine a plurality ofradio resources reserved for a further radio terminal, in particular aset of SPS resources in dependence on the decoded SCI; and determine thecontention indicator to represent no contention, if the second subset ofthe set of SPS radio resources and the determined plurality of radioresources are disjoint.

Accordingly, if the SCI information is used to determine that the secondsubset of SPS radio resources can be used to resume and continue thedata transmission.

According to an advantageous example, the determining the contentionindicator comprises: determine a received signal strength for the atleast one sensing resource; compare the received signal strength with athreshold; and determine the contention indicator in dependence on thecomparison.

If the signal strength is above the threshold, the first radio terminaldetermines a contention situation and can advantageously react in orderto overcome a potential resource conflict.

According to an advantageous example, the sensing comprises: sensing afirst part of the sensing resource; and transmitting, towards the atleast one second radio terminal, data in a second part of the sensingresource.

Advantageously, sub-slot sensing and transmission increases thepossibility of a data transmission. Moreover, the sensing is includedand could even be sufficient to perform SCI decoding, if the first partcomprises an SCI.

According to an advantageous example, the refraining comprises:determine, after the SPS configuration has lapsed, a further SPSconfiguration for the first radio terminal, wherein the further SPSconfiguration indicates a further set of further SPS radio resources ofthe shared device-to-device radio channel, in particular of the sidelinkchannel, SL-CH, wherein the indicated further SPS radio resources areintended for a transmission by the first radio terminal; and starttransmitting, towards the at least one second radio terminal, data viathe further set of SPS radio resources if a time period since thesensing of the at least one sensing resource and before the firsttransmission via the further set of SPS radio resources has not lapsed.

Therefore, depending on the QOS requirements, the data transmission iscontinued. The time period since the sensing guarantees the QoSrequirement.

According to an advantageous example, the first radio terminal isfurther configured to: transmit, towards the at least one second radioterminal, the contention indicator, wherein the transmitted contentionindicator indicates a contention situation at the at least one sensingresource and the identity of the at least one sensing resource.

Advantageously, the radio terminals in the vicinity have knowledge ofthe contention situation on the at least one sensing resource and mayomit a transmission on this resource.

According to an advantageous example, the data transmitted towards theat least one second radio terminal comprises a reservation indicator,which indicates an identity of the subsequent SPS radio resource orsubsequent auxiliary radio resource for transmitting by the first radioterminal.

By distributing the identity of the subsequent SPS radio resource or thesubsequent auxiliary radio resource, the at least one second radioterminal is able to refrain from using these resources for transmission.

According to an advantageous example, the data transmitted towards theat least one second radio terminal comprises a sensing indicator, whichindicates an identity of the subsequent SPS radio resource for sensingby the first radio terminal.

By distributing the identity of the subsequent sensing resource, the atleast one second radio terminal is able to refrain from using theseresources for transmission.

According to a second aspect of the description, a method to operate afirst radio terminal of a radio communications network, in particular aV2X UE of a V2X network is provided, wherein the method comprises:determining a SPS configuration for the first radio terminal, whereinthe SPS configuration indicates a set of SPS, semi-persistentlyscheduled, radio resources of a shared device-to-device radio channel,in particular of a sidelink channel, SL-CH, wherein the indicated SPSradio resources are intended for a transmission by the first radioterminal; transmitting, towards at least one second radio terminal, datavia a first subset of the set of the SPS radio resources; selecting atleast one sensing resource out of the set of SPS radio resources;sensing the at least one sensing resource; determining a contentionindicator in dependence on the sensing of the at least one sensingresource; and refraining from transmitting data via a second subset ofthe set of SPS radio resources if the contention indicator indicates acontention situation at the at least one sensing resource.

According to a third aspect of the description, a method to operate aradio communications network is provided, wherein the radiocommunications network comprises the first radio terminal according tothe first aspect and the second radio terminal.

BRIEF DESCRIPTION OF THE DRAWINGS

In the figures,

FIG. 1 depicts a schematical flow diagram for operating a first radioterminal;

FIGS. 2 to 5 each depict a schematical sequence diagram for operating aradio communications network;

FIG. 6 depicts a problem of the prior art; and

FIG. 7 depicts a structure of the first radio terminal.

DETAILED DESCRIPTION

FIG. 1 depicts a schematical flow diagram for operating a first radioterminal of a radio communications network, in particular for operatinga V2X UE of a V2X network. According to a step 102, the first radioterminal determines a SPS configuration for the first radio terminal,wherein the SPS configuration indicates a set of SPS, semi-persistentlyscheduled, radio resources of a shared device-to-device radio channel,in particular of a sidelink channel, SL-CH. The indicated SPS radioresources are intended for a transmission by the first radio terminal.

The sidelink channel SL-CH comprises, for example, a physical sidelinkshared channel, PSSCH, and a physical sidelink control channel, PSCCH.Sidelink describes a UE to UE interface for sidelink communication,sidelink discovery and V2X sidelink communication. The sidelinkcorresponds to a PC5 interface for sidelink communication and sidelinkdiscovery, and V2X sidelink communication. Sidelink communicationcomprises functionality enabling ProSe Direct Communication between twoor more nearby UEs, using E-UTRA technology but not traversing anynetwork node.

The determination according to step 102 is done by a central schedulingunit or in a distributed way. Therefore, according to the example of adistributed scheduling, the step 102 comprises a determination of thetransmission opportunities in the form of the SPS configurationincluding the set of SPS resources rr1-rr4. The determination accordingto step 102 of the distributed scheme may comprise sensing resources tobe free or occupied.

According to a step 104, the first radio terminal transmits, towards atleast one second radio terminal, data via a first subset of the set ofthe SPS radio resources. The first radio terminal may reserve every timeonly one subsequent look ahead SPS radio resource, i.e., eventually fora next future transmission. If a stopping condition is triggered as in astep 106, the first radio terminal skips data transmission either thenext transmission opportunity or a subsequent next transmissionopportunity with a time maximum. The latter may be allowed to fulfil aQoS, latency requirement, or UE capabilities like processing time.

According to the step 106, the first radio terminal selects at least onesensing resource out of the set of SPS radio resources.

According to a step 108, the first radio terminal senses the at leastone sensing resource.

According to a step 110, the first radio terminal determines acontention indicator in dependence on the sensing of the at least onesensing resource.

According to a step 114, the first radio terminal, refrains fromtransmitting data via a second subset of the set of SPS radio resourcesif the contention indicator indicates a contention situation at the atleast one sensing resource. The step 114 comprises to clear theconfigured sidelink grant at the end of the corresponding SC, SidelinkControl, Period. In other words, an SPS release for the future SPS radioresources is conducted.

The SC Period consists of a transmission of SCI, Sidelink ControlInformation, and its corresponding data. The SCI contains the sidelinkscheduling information such as resource block assignment, modulation andcoding scheme, Group Destination ID (for sidelink communication) andPPPP, ProSe Per-Packet Priority, (for V2X sidelink communication),

The data transmitted towards the at least one second radio terminalcomprises control information and payload.

According to an example, the data transmitted towards the at least onesecond radio terminal comprises a reservation indicator, which indicatesan identity of the subsequent SPS radio resource or subsequent auxiliaryradio resource for transmitting by the first radio terminal.

According to an example, the data transmitted towards the at least onesecond radio terminal comprises a sensing indicator, which indicates anidentity of the subsequent SPS radio resource for sensing by the firstradio terminal.

The selection of the sensing resource according to step 106 isexemplified in the following. According to an example, the selectionprocedure is configured via, e.g., SIB acquisition, e.g., SIBX, SIBY,SIB21, SIB 26 or via dedicated RRC messages. According to an example,the selection information about the location and selection procedure ofthe sensing resource is configured in the resource pool configurationinformation element, e.g., in the Sidelink BWP configuration, and or SLcarrier configuration. A cell may activate or deactivate the operationduring SIB acquisition or re-acquisition or during RRC reconfigurationor configuration.

Skipping a transmission of the configured SPS radio resources andsensing the sensing resource instead is referred to as skipping in thefollowing. A triggering event for determining the sensing resource isreferred to in the following in relation with a timer or counter.

When the SIB is acquired with either a configuration of the skippingand/or a configuration of a timer, or the activation to perform theskipping operation or the activation to execute of the saidtimer/counter operation, the first radio terminal UE1 is expected toperform skipping according to the (pre-)configured timer/counter valuesas specified before.

According to an example, in the covered case, the network provides thetimer/counter values, for example via a RRC configuration received bythe radio terminal.

According to a further example, in the out-of-coverage case, the UE usesits pre-configured configuration of the timer/counter values or aninformation element received via a sidelink control channel.

For example, a SIB (e.g., SIBX, SIBY, SIBZ etc.) includessl-ConfigCommon, which, e.g., may contain a sl-BWP-Config and/orSL-BWP-Pool-Config or sl-ResourcePool-Config information elements, whichcontain the configuration and/or activation/deactivation of the saidfields, i.e., in the TX resource pool(s) configuration.

For example, when a dedicated RRC message configures either the skippingmechanism or the skipping counter/timer, the values of thetimer/counters and/or the procedure for the skipping, e.g., sub-slot,delayed skipping, early skipping, etc., are configured. If not, theconfiguration in the SIB information and/or configuration inSL-preconfiguration messages may be used.

According to an example, a cell-manager, for example residing in a basestation, decides to activate or deactivate the saidoperations/timers/counters. Hence, the first radio terminal checks theRRCReconfigurationSidelink (or any other SL RRC Configuration element)or the SIB TX pool configuration (or any configuration element) for atleast one of the following fields:

SL-ConfiguredResourcesSkippingActivation/Deactivation, e.g., Boolean{TRUE or FALSE}; SL-ConfiguredResourcesSkippingMode, e.g., enumerate{EarlySkipping, LateSkipping, SubSlot-Skipping (first or last) }; SL-ConfiguredResourcesSkippingTimerActivation/Deactivation or SL-ConfiguredResourcesSkippingCounterActivation/Deactivation orSL-ConfiguredResourcesSkippingTimer/SL-ConfiguredResourcesSkippingCounter, where the timer may be enumerate {ms20, ms50, ms100, ms200, ms300, ms400, ms600, ms1000, ms1500, ms2000}the counter may be enumerate {N5, N10, N20, N30, N40, N60, . . . etc.}.

If the first radio terminal UE1 is configured by upper layers (e.g., byRRC dedicated messages or during SIB (e.g., SIBX, SIBY, SIBZ, . . . )acquisition) to perform an out of sequence transmission. The first radioterminal UE1 checks if the use of out-of-sequence operation is “enabled”or “disabled” after finding the skipping and/or the skipping timerconfiguration. If the above skipping operation or skipping timer/counterare configured and “enabled”, the first radio terminal UE1 examines theacquired SIB, e.g., SIBX, SIBY, SIBZ, etc., if it contains (in one ofits information elements, e.g., SL-BWP-PoolConfig) orRRCReconfigurationSidelink if it contains (in one of its informationelements)), for example:SL-ConfiguredResourcesOutOfSequenceEnabled/SL-ConfiguredResourcesOutOfSequenceEnabled(or one field, e.g., SL-ConfiguredResourcesOutOfSequence with Boolean{TRUE or FALSE}), and SL-ConfiguredResourcesOutOfSequenceMode, e.g.,enumerate {AfterSkipping, BeforeSkipping}.

According to an example, if the out of sequence is configured, a lowerlayer, e.g., MAC, may generate a MAC PDU that contains: a MAC controlelement, CE, with one logical channel ID (LCID) containing thefollowing:

-   -   1 (or more bits) Informing one or more stations that this        current message is an out-of-sequence message for a configured        resources;    -   If the out of sequence message is generated after skipping        (e.g., when        SL-ConfiguredResourcesOutOfSequenceMode=AfterSkipping}, this MAC        CE may contain N-bits informing other station about: whether the        SPS will continue for another (one or more) periods; and        Interference station layer 2 ID or Layer 1 D and or        SPS/configured resource ID of the skipped process.

FIG. 2 depicts a schematical sequence diagram for operating the radiocommunications network RCN. According to an arrow 200, the focus of theillustration is on a data transmission from the first radio terminal UE1to the second radio terminal UE2.

The first subset ss1 is used in steps 104, 105 to transmit data towardsthe second radio terminal UE2.

The selection of the step 106 of the at least one sensing resource srcomprises to select the at least one sensing resource sr out of the sets1 of SPS resources rr1-rr4 upon expiry of a time period T1 since thefirst transmission via the set of SPS resources rr1-rr4 or upon reachinga number of transmissions conducted via the set s1 of SPS resourcesrr1-rr4.

According to an example, the time period T1 is determined based on apredefined rule taking into account a resource reselection counterSL_RESOURCE_RESELECTION_COUNTER and a configurable thresholdprobResourceKeep for a probability of keeping SPS radio resources. Thethreshold probResourceKeep is selected by upper layers to maintain theSPS radio resources, which are initially selected for periodictransmission longer after the expiration of theSL_RESOURCE_RESELECTION_COUNTER. When the radio terminal UE1 reserves afinite number of SPS radio resources, the predefined rule is applied. Asa specific example of the predefined rule, the number of subframes inone set of time and frequency resources related to transmissionopportunities of the PSSCH may be given as a specific value (e.g., Cresel). At this time, the specific value C resel may be defined as10*SL_RESOURCE_RESELECTION_COUNTER (when a specific counter (e.g.SL_RESOURCE_RESELECTION_COUNTER) is set), otherwise C resel may be setto 1 (for example if SL_RESOURCE_RESELECTION_COUNTER is not set). Arandom value of SL_RESOURCE_RESELECTION_COUNTER of 5 or more and 15 orless may be set. For example, when SL_RESOURCE_RESELECTION_COUNTER is 5,a total of 50 subframes may be reserved for transmission of PSSCH. Forexample, when SL_RESOURCE_RESELECTION_COUNTER is 15, a total of 150subframes may be reserved for transmission of PSSCH. After reaching theend of these reserved SPS resources, the radio terminal UE1 determineswhether to select the next resource, following the previously determinedSPS scheme, as the sensing resource sr by comparing a determined randomnumber and the threshold probResourceKeep.

The time period T1 or the number of transmissions is determined independence on at least one of the following: a measured channel load ofthe physical device-to-device channel, a present status of an egressqueue of the first radio terminal UE1, and a QoS indicator. According toan example, the time period T1 is selected from a set of preconfiguredtime periods. According to an example, the time period T1 is configuredvia the network. According to an example, the time period T1 is fixed.

According to the present example, in step 108 the first radio terminalUE1 senses an occupation of the sensing resource sr, as a third radioterminal UE3 is transmitting at least via a part of the sensing resourcesr. Therefore, the sensing of the step 108 comprises: sensing a firstpart of the sensing resource sr; and transmitting, towards the at leastone second radio terminal UE2, data in a second part of the sensingresource sr.

Therefore, if sub-slot operation is allowed, the first radio terminalUE1 skips only a sub-slot in the sense of the first part of configuredSPS radio resource allocated after which a triggering condition isvalid. For example, the first radio terminal UE1 may perform at leastone of the following:

-   -   defer transmission to the end of the slot in the sense of the        second part, therefore leaving the beginning symbols of the slot        free for sensing;    -   an early transmission at the beginning of slot, in the sense of        the second part, and leaving enough symbols in remaining time of        the slot to allow sensing on such symbols, in the sense of the        first part.

In both examples, the first radio terminal UE1 is able to transmit onparts of the slot. In these cases, the first radio terminal UE1 has toinclude its SCI for other radio terminals UE2, UE3 to sense and decodesaid transmission. Either a defer or an early transmission within a slothas to include in the SCI enough information to identify at least: thenext transmission reservation, an indication of a partial slot skipping.In both examples, it is up to the first radio terminal UE1 to do eithera segmentation of the resources (as the transmission block size has tobe reduced to the amount of resources in the sub-slot duration) or todrop the data that cannot be fitted in the sub-slot transmission. If theUE is requested to sense SCI and its associated RSRP, then the UE mayselect to defer the transmission to the end of the slot. However, ifRSSI sensing is allowed, both examples are suitable for grant skippingand sensing.

If the first radio terminal UE1 performed a sub-slot resource skippingand identified a clear channel (i.e., no collision or half-duplexdetected), the first radio terminal UE1 resumes transmitting via theconfigured SPS radio resources. The advantage of having only sub-slotskipping is to allow reservation continuity, where other radio terminalsUE2 and UE3 will consider the first radio terminal UE1 always transmitson the indicated SPS radio resources. However, in a full slot skipping,and if not cross-reserved by other transmission, other UEs may assumecomplete grant skipping and pre-empt the said UE reservations. This maybe avoided by partial slot skipping.

For partial slot skipping, every time the first radio terminal UE1performs the partial skipping, the first radio terminal UE1 avoidstransmission at locations, in particular of the skipped first part: Atthe beginning, i.e., to decode possible colliding SCI or the associatedPDCCH/PDSCH DMRS; Another part of the slot (not necessarily the firstpart), where the first radio node UE1 may skip possible DMRS locations(for correlation detection) or RSSI computation; and the first radioterminal UE1 may decide to skip any part of the slot and decide to doenergy detection.

According to an example, the step 110 of determining the contentionindicator CONT, not shown in FIG. 2, comprises: determine a receivedsignal strength for the at least one sensing resource sr; compare thereceived signal strength with a threshold; and determine the contentionindicator CONT in dependence on the comparison.

According to the step 114, the radio terminal UE1 decides to refrainfrom using the already scheduled resources rr3 and rr4 in order to avoidcollisions on the radio channel CH.

According to an example, the refraining 114 comprises: determine,according to a step 202, after the SPS configuration has lapsed, afurther SPS configuration for the first radio terminal UE1, wherein thefurther SPS configuration indicates a further set fs of further SPSradio resources rr10, rr11 of the shared device-to-device radio channelCH, in particular of the sidelink channel, SL-CH, wherein the indicatedfurther SPS radio resources rr10, rr11 are intended for a transmissionby the first radio terminal UE1; and start transmitting, according tosteps 416, 418, towards the at least one second radio terminal UE2, datavia the further set fs of SPS radio resources rr10, rr11 if a timeperiod T2 since the sensing 108 of the at least one sensing resource srand before the first transmission via the further set of SPS radioresources rr10, rr11 has not lapsed. According to an example, the timeperiod T2 is selected from a set of preconfigured time periods.According to an example, the time period T2 is configured via thenetwork. According to an example, the time period T2 is fixed.

The first radio terminal UE1 may drop a transmission of data in itsegress queue or postpone the transmission of the data and wait for theresources rr10 and rr11. However, a transmission of data in the egressqueue is skipped in the sidelink transmission granted resource in formof the sensing resource sr. Accordingly, the first radio terminal UE isable to listen to other colliding transmitters like UE3, i.e.,transmitter coexisting on the same reserved periodic resources.

According to an example, the first radio terminal UE skips one (or more)configured sidelink SPS resources, when triggered by expiry of the timerT1 exceeding a pre-configured threshold, value or transmits aconsecutive number of periods of the configured sidelink resources. Arelation between the timer T1, the number of consecutive transmissionsafter the sensing resource sr, and a reselection counter for reselectingthe radio resources of the SPS configuration and a Probability ofResource Keep can be configured. According to an example, the timer T1can start once the SPS starts and allows to be triggered after the firstreselection counter goes to 1 and/or a probability of resource keep isaccepting periodic extension.

According to an example, based on the QoS or latency requirement, thefirst radio terminal UE1 extends a transmission for at least anotherconfigured time/frequency resources like rr10 and rr11, after thetriggering condition for determining the sensing resource sr, and delaythe skipped transmission until a following subsequent configured grantfor radio resources rr10, rr11.

According to an example, sub-slot transmission is allowed and the UEskips only a part of the slot to perform sensing. E.g., the first partof a slot can be skipped to be able to decode the SCI decoding and thecorresponding RSRP measurements. E.g., the second part of a slot can beskipped to perform RSSI measurement only. E.g., any part of the slot andrather do an energy detection on the skipped part. If the radio terminalUE1 identifies a resource collision or half-duplex problem, the radioterminal UE1 decides to cancel all remaining configured SPS resourcesrr3, rr4 and delete the existing configured SPS reservations.

The condition to determine the sensing resource sr from the SPS radioresources comprises at least one of the following:

-   -   The timer T1, which starts after a first transmission of a        periodic transmission opportunities, expires before an upcoming        transmission opportunity; and/or    -   A counter, which counts the number of consecutive transmission        made in the configured resource set s1, spaced with a        periodicity.

In order to identify which resources are selected to be sensed insteadof being used for transmission, the first radio terminal UE1 selects thefollowing transmission opportunity/opportunities: A subsequenttransmission opportunity/opportunities that directly follows thetriggering event described before; a subsequent transmissionopportunity/opportunities that follows one or more transmission of theconfigured SPS radio resources that can be transmitted following thetriggering event and not longer than a relax time period;

After the first radio terminal UE1 decided to start sensing on thesensing resource sr, the first radio terminal UE1 selects only oneconfigured SPS radio resource for sensing. During the time identifiedfor the at least one sensing resource sr, the UE switches it receiver onand performs sensing, which can comprise SCI decoding.

According to an example, the first radio terminal UE1 performs sensing(once the triggering condition and event are satisfied) on multipleconfigured sidelink transmission opportunities. These opportunities maybe: A periodic sequence of the configured sidelink granted transmissionopportunities; A subset of resources selected randomly from the originaltransmission opportunities; A quasi-periodic set of transmissionopportunities which may follow a pattern or they may be chosen based on,e.g. the traffic density of the first radio device UE1, the arrival rateof MAC PDUs at the first radio terminal UE1, and/or a number of the MACPDUs left in the egress queue of the first radio terminal UE1.

FIG. 3 depicts a schematical sequence diagram for operating the radiocommunications network RCN.

According to an example, the step 110 of determining the contentionindicator CONT comprises: decode SCI, Sidelink Control Information,present in the received at least one sensing resource sr; determine aplurality of radio resources r2_UE2, r3_UE3 reserved for a further radioterminal UE3, in particular a set s3 of SPS resources r2_UE2, r3_UE3 independence on the decoded SCI; and determine the contention indicatorCONT to represent no contention, if the second subset ss2 of the set s1of SPS radio resources rr1-rr4 and the determined plurality of radioresources r2_UE3, r3_UE3 are disjoint. Decoding the SCI may result indetermining the source ID of the transmitting third radio terminal UE3,and/or a destination ID of the transmission.

According to steps 112, 116, the first radio terminal UE1 resumes itstransmission and transmits, towards the at least one second radioterminal UE2, data via the second subset ss2 of the set s1 of SPS radioresources rr1-rr4 if the contention indicator CONT indicates nocontention situation at the at least one sensing resource sr.

Once the evaluation results in a positive collision or half-duplexproblem, the first radio terminal UE1 sets the contention indicator CONTto parametrize the outcome of the evaluation process, such that:SL_resourceContention=TRUE. In the same example, if the evaluation isnot identifying any collision and/or half-duplex issue, the contentionindicator CONT stays as the default value “FALSE”.

According to an example, if the SL_resourceContention is set to TRUE,the first radio terminal UE1 triggers the reselection procedure if theUE1 is allocated to those sidelink resources using an autonomousresource mode, i.e., mode 2, or the UE1 sends to the network configuredgrant reconfiguration request. Additionally, the UE1 deletes anyremaining configured sidelink grants associated with the said periodictransmission.

The first radio terminal UE1 that skipped one transmission for sensingat sensing resource sr, transmits the contention indicator CONT to theother radio terminals UE2, UE3 in the vicinity of the first radioterminal UE1, announcing possible collision or half-duplex problem. Inthis case, the first radio terminal UE1 may inform only the conflictingradio terminal UE3, i.e., UE1 existing in the same SPS reserved periodwhere the said UE1 is suspecting UE3 to be colliding or overlapping withits own resources. A colliding radio resource or half-duplex situationmay be detected based on: SCI decoding and/or RSRP measurements, RSSIdecoding; Physical layer ID (L1 ID) resolution, e.g., if the one or moreof the UEs is transmitting in the same time-slot (a half-duplex issue)and the L1-ID are within the UE L1 ID list; Layer-2 ID (L1 ID)resolution, e.g., if the one or more of the UEs is transmitting in thesame time-slot (a half-duplex issue) and the L1-ID are within the UE L1ID list. Based on this resolution (and the UE decoding capability andimplementation), the first radio terminal UE1 identifies if resourcesare colliding/overlapping with that of other UEs.

Hence, if the first radio terminal UE1 detects a contention situationwith colliding or overlapping resources, as a consequence of resolvingthe situation the first radio terminal UE1 transmits an theout-of-sequence transmission such that:

-   -   The first radio terminal UE1 sends the said out-of-sequence        message via at least one auxiliary radio resource ar1, ar2 in a        unicast transmission (i.e., to the identified UE who rises the        collision/overlapping problem).    -   The first radio terminal UE1 transmits said out-of-sequence        message in a groupcast transmission (i.e., to those identified        UEs who rise the collision/overlapping problem). In this case,        those UEs have to reside in a certain communication range. This        collision/half-duplex situation resolution communication range        is configured for the first radio terminal UE1. Hence, any UE        may exist outside this communication range may not be requested        to fulfil resolution to the said collision/half-duplex        situation.    -   The first radio terminal UE1 send the said out-of-sequence        message in a broadcast transmission (i.e., to all UEs who exist        in the vicinity of the said UE). In this case, the UE may not be        able to identify the colliding/overlapping UE-ID(s).

In all communication cast types, unicast, groupcast, and broadcast, theout-of-sequence control information may be carried via a physicalsidelink control information (SCI) or a PC5 (i.e., sidelink interface)radio resource control (RRC) signaling.

According to an example, the first radio terminal UE1 transmits on eachtransmission opportunity according to the SPS radio resources, at least,an initial transmission and one more redundancy transmission version,which are, for example, another code version or the same copy of theinitial transmission. In each of these transmissions, there could bereservations including one or more of the following cases:

-   -   For Initial transmissions, there could be reservation for        another redundancy version associated with the initial        transmission, and a reservation for another initial subsequent        transmissions;    -   Redundancy version(s) only reserve subsequent transmission of        similar redundancy versions, e.g., of the same levels, e.g.,        RV1, of future subsequent re-transmission;    -   Initial transmission and redundancy versions reserves each        other's resources also in subsequent transmission opportunity in        a chain-based reservation.

In the last case as stated above, if only one configured time/frequencyresources is skipped, there will be at least one reservation remainingfor a subsequent transmission following the skipped transmission slot.

A data packet is generated by upper layers and segmented at the RLClayer (after PDCP packing). One segment is referred to in a lower layer(from MAC on) as a transport block (TB). Transport block size (TBS) isgenerally computed after the physical layer modulation and coding schemeselection (i.e., this mean it includes information data and some paritybits. Finally, a cyclic redundancy check is added to this blockrepresenting the redundancy version, which is composed of: someinformation data, some parity bits, and CRC. The CRC itself is afunction of the other two data parts (information and parity). Since thechannel coding is using generally low modulation coding scheme depth, acircular buffer is used to sort the information data and all possibleparity bits. In the first redundancy version, some more information bitsand some parity bits are selected from the circular buffer. If anotherre-transmission is needed (either for blind re-transmission orAcknowledgement based retransmissions), another redundancy version isused. In this case, this new (2nd) redundancy version is consideringsome (other) information bits+some (other) parity bits, which is not thesame values as the first transmission for example. Similar to the firstversion of the redundancy versions, a CRC block is combined to theencoded TB.

Each redundancy version refers to part of the information bits and partof the parity bits arranged in a circular buffer. The CRC result foreach redundancy version, e.g., according the CRC function polynomialfunction provided, is appended to each redundancy version bit formingthe complete Transport Block size, i.e., CRC of each redundancy versionis different.

FIG. 4 depicts a schematical sequence diagram for operating the radiocommunications network RCN.

According to a step 402, the first radio terminal UE1 determines atleast one auxiliary radio resource ar1, ar2, wherein the at least oneauxiliary radio resource ar1, ar2 and the set s1 of SPS radio resourcesrr1-rr4 are disjoint.

According to an example, the step 402 comprises to randomly select thetime and frequency resources for SL-SCH and SCI of a sidelink grant fromthe selected resource pool. The random function shall be such that eachof the allowed selections can be chosen with equal probability.

According to steps 404, 406, the first radio terminal UE1 transmits,towards the at least one second radio terminal UE2, data via the atleast one auxiliary radio resource ar1; ar2.

According to the step 404, the data transmitted via the at least oneauxiliary radio resource ar1; ar2 comprises the out-of-sequenceindicator OOS, which indicates that the auxiliary radio resource ar1;ar2 is an auxiliary radio resource ar1; ar2.

According to the step 406, the first radio terminal UE1 transmits,towards the at least one second radio terminal UE2, the contentionindicator CONT, wherein the transmitted contention indicator CONTindicates a contention situation at the at least one sensing resource srand the identity of the at least one sensing resource sr.

According to an example, the refraining, according to the step 114,comprises: determine, according to a step 412, after the SPSconfiguration has lapsed, a further SPS configuration for the firstradio terminal UE1, wherein the further SPS configuration indicates afurther set fs of further SPS radio resources rr10, rr11 of the shareddevice-to-device radio channel CH, in particular of the sidelinkchannel, SL-CH, wherein the indicated further SPS radio resources rr10,rr11 are intended for a transmission by the first radio terminal UE1;and start transmitting, according to steps 416, 418, towards the atleast one second radio terminal UE2, data via the further set fs of SPSradio resources rr10, rr11 if the time period T2 since the sensing 108of the at least one sensing resource sr and before the firsttransmission via the further set of SPS radio resources rr10, rr11 hasnot lapsed.

The auxiliary radio resources ar1, ar2 are located before and after thesensing resource sr for performing sensing measurement. Of course, onlyone of the auxiliary radio resources ar1, ar2 can be used to transmitdata towards the second radio terminal UE2.

According to an example, the first radio terminal UE1 selects, accordingto step 402, an out-of-sequence-transmission time/frequency resource inthe sense of an auxiliary radio resource ar1, ar2 within a selectionwindow of a length T slots, which does not include any of the configuredSPS sidelink resources rr1 to rr4.

According to an example, the first radio terminal UE1 extendstransmission of the data, the first radio terminal UE1 transmitting viathe at least one auxiliary radio resource ar1, ar2, which may bereserved by the next configured resources following the triggeringcondition.

Once the triggering condition arises, the first radio terminal UE1selects from a new or an old configured or preconfigured sensing windowradio resources for transmission, which could be:

-   -   single radio resource transmitted once in a selection window        directly after the triggering event and before the skipped        transmission opportunity in the sense of the sensing resource        sr;    -   a single radio resource transmitted once in the selection window        directly after the triggering event and after skipping the next        possible transmission opportunity in the sense of the sensing        resource sr after the said triggering event;    -   a single radio resource (transmitted in more than one selection        window, i.e., current following the event and the following        ones. This could be sent before or after a skipped transmission        opportunity;    -   resources within the same selection window, i.e., following a        transmission time/frequency patterns. Those resources or those        sub-resources can be used to guarantee enough transmission        opportunities for other UEs to detect the out of sequence        situation.

According to an example, if retransmission is activated, for exampleHARQ retransmission, the first radio terminal UE1 selects, according tostep 402, at least one of the retransmission opportunities, occurringbefore or after a skipped configured sidelink grant, to be the at leastone auxiliary radio resource ar1, ar2.

According to an example, the first radio terminal UE1 randomizes theposition of the at least one auxiliary radio resource ar1, ar2 if thedata is transmitted more than once during the time of configured SPSsidelink resources rr1-rr4.

According to an example, the data transmitted via the at least oneauxiliary radio resource ar1, ar3, in particular comprises apreconfigured or configurable pattern if the data is transmitted morethan once during the time of a configured sidelink resources in order toidentify that the data transmission is repeated.

The data transmitted via the at least one auxiliary radio resource ar1,ar2, in particular out-of-sequence transmission content, the messagesidelink control information (SCI) sent over physical control channel orthe message control channel sent over the shared data channel, comprisesat least one of the following:

-   -   An identifier or a field or at least one bit in the sidelink        control information indicating that this transmission is an        out-of-sequence of a configured (periodic) sidelink resources,        wherein this field for collision can be either exist “TRUE” or        not “FALSE”; additionally, an identifier to the colliding        resources (as reserved in Time/frequency) or an identifier to        the SPS/Configured-grant index (if exist) that is        contention-affected;    -   For preserving the original configured resources, a reservation        for the skipped (and canceled) transmission slot in the sense of        the sensing resource sr;    -   A reservation to a subsequent transmission within the configured        SPS radio resources rr3, rr4, where the first radio terminal UE1        is intending to resume the configured (periodic) transmission;        If the UE1 wants to resume (e.g., after indicating a resuming        intention), and the UE1 decided to resume on the same,        previously reserved, SPS/configured-grant resources, then the        UE1 is requested to send the reservation to a subsequent        transmission within the same configured resources where the UE1        is intending to resume the same configured (periodic) SPS        transmission. This is important to those UEs who can only decode        the out of sequence transmission;    -   Else, if the UE1 wants to resume (e.g., after indicating a        resuming intention), and the UE1 decided to resume on time        frequency resources, then the UE1 is requested to send a new        periodic reservation interval for the said configured resources        to be used after when the UE1 indicates continuation for the        (periodic) transmission;    -   the QoS of the said UE transmission, or the new QoS of the        continuation of the said transmission sequence, i.e., if the UE        has to increase the QoS of the said SPS transmission to resolve        the collision    -   The periodic reservation interval for the said configured SPS        radio resources rr3, rr4 to be used after when the first radio        terminal UE1 indicates continuation for the (periodic)        transmission; and    -   A cancelation to the remaining configured resources rr3, rr4 if        the first radio terminal UE1 identified a possible collision, or        resource contention of a higher QoS transmission of another UE        or half-duplex problem.

The first radio terminal UE1 informs other radio terminals, for examplethe radio terminal UE3, about their collision or half-duplex problem bytransmitting a corresponding indicator like OOS towards the at least oneradio terminal UE3 in the vicinity of the first radio terminal UE1.According to an example, the first radio terminal UE1 sends a unicastsidelink control information or sidelink radio resource control (RRC)information to the colliding/half-duplex UE3. According to anotherexample, the first radio terminal UE1 sends a groupcast sidelink controlinformation or groupcast sidelink radio resource control (RRC) to allpossible colliding/half-duplex UEs or UEs with a communication range ofthe first radio terminal UE1. This collision/half-duplex problemresolution communication range can be configured/pre-configured for thefirst radio terminal UE1. According to another example, the first radioterminal UE1 sends a broadcast sidelink control information or broadcastsidelink radio resource control (RRC) information or sidelink masterinformation block to all UEs in the field.

The control information of the data transmitted via the auxiliary radioresources ar1, ar2 includes an identifier to collision/half-duplexproblem on a said reservation period/SPS-index (if exist), the QoS ofthe said UE transmission, the intention of UE1 to resume or to cancelthe said transmission.

According to an example, once the first radio terminal UE1 starts thetransmission via the auxiliary radio resources ar1, ar2 periodically inmultiple selection windows, the first radio terminal UE1 transmitsmultiple out-of-sequence transmissions until it resumes the originalconfigured SPS resources as exemplified in FIG. 5. Hence, if the firstradio terminal UE1 transmits more than one out-of-sequence transmissionbefore resuming the transmission via the granted SPS radio resources,the first radio terminal UE1 may also be configured to select: randomradio resources for each out-of-sequence transmission (the multipleout-of-sequence transmissions), or a set of resources following aconfigured time/frequency resource pattern map for the multipleout-of-sequence transmissions. However, if an N consecutive configuredresources elapse, the first radio terminal UE1 performs a reselectionand deletes the originally reserved SPS radio resources. In anotherexample, the first radio terminal UE1 selects to perform out-of-sequencetransmissions until a completely new configured SPS radio resources areselected.

For selecting the out-of-sequence resources, the selection window of alength T_slots may be considered for this process as follows, either:the same window size and location as the original SPS/configuredsidelink resources selection window, or a window different from theoriginal SPS/configured sidelink resources selection window, which canalso be considered to either include any of the reserved configuredsidelink resources, but exclude them during resource selection, or to bea short selection window that exclude all originally reserved resources.In the latter option, i.e., the sensing window for out-of-sequencetransmission(s) does not include the original resources; the first radioterminal UE1 may consider a short sensing window for this operation.

According to an example, if the first radio terminal UE1 is extendingits transmission on its originally reserved SPS resources according toFIG. 5, e.g., for QoS or stringent latency requirements, the first radioterminal UE1 may only consider continuing the original resourcereservation no more than a T_relax slots elapse. Hence, after atriggering event plus a time T2 (with a maximum value T_relax), theradio terminal UE1 performs grant skipping and allows sensing in such asituation. Additionally, the first radio terminal UE1 transmits anout-of-sequence transmission via the auxiliary radio resources ar1, ar2as described above.

FIG. 5 depicts a schematical sequence diagram for operating the radiocommunications network RCN.

According to a step 502, the first radio terminal UE1 determines the atleast one auxiliary radio resource ar1, ar2, wherein the at least oneauxiliary radio resource ar1, ar2 and the set s1 of SPS radio resourcesrr1-rr4 are disjoint.

According to steps 504, 506, the first radio terminal UE1 transmits,towards the at least one second radio terminal UE2, data via the atleast one auxiliary radio resource ar1; ar2.

According to an example, the first radio terminal UE1 is configured tosend data via more than one auxiliary radio resources ar1 and ar2 beforeresuming the transmission via the SPS resources rr3 and rr4. In thiscase, the first radio terminal UE1 is configured to: Select randomauxiliary radio resources ar1, ar2 for each out-of-sequencetransmission, and/or select a resource pattern for the auxiliary radioresources ar1, ar2.

FIG. 6 depicts a problem of the prior art. For periodic datatransmission in Mode 2 resource allocation, semi-persistent schedulingSPS is used. In this case, the UEA performs a sensing mechanism wherethe UEA selects transmission resources in time and frequency. Similaridentical copy of these resources (in both time and frequency) arerepeated at a period selected from enumerated (configured orpre-configured values). Once the initial transmission is selected, theUEA adds to its sidelink control information (SCI) the time resources(e.g., slot index in a resource-pool), the frequency resources (e.g., asa subchannels start index and the number of contiguous N subchannels),and the reservation period P_resv. The initial transmission does notindicate for how many periods are these resources reserved.

UEB being able to successfully decode the initial SCI (or any subsequentSCI) can indicate those resources to be reserved for multiple periods.

However, if UEC cannot successfully receive the initial SCI or thesubsequent SCIs on the physical sidelink control channel, PSCCH, UECwill fail to indicate those resources as reserved. In this case, UE3 isallowed to reselect those resources for their own transmission producingcollisions on the physical sidelink shared channel, PSSCH.

UE3 may fail to decode an SCI if any of these cases occur: Thetransmission blocks (including the intended SCI) is overlapping with theUE's own transmission in the same time and frequency resources; Thetransmission blocks (including the intended SCI) is overlapping with theUE's own transmission in the same time resources but different frequencyresources; Due to channel path-loss and propagation errors.

FIG. 7 depicts a structure of the radio terminal UE1. Said radioterminal UE1 comprises at least one processor P, at least one memory Mcomprising computer program code CP, at least one communication moduleC, and at least one antenna A, wherein the computer program code CP isconfigured to interact with the at least one processor P, the at leastone communication module C, and the at least one antenna A to cause theradio terminal UE1 to conduct the methods described above.

The invention claimed is:
 1. A first radio terminal (UE1) of a radiocommunications network (RCN), said first radio terminal (UE1)comprising: at least one processor, at least one memory comprisingcomputer program code, at least one communication module, and at leastone antenna, wherein the computer program code and the memory areconfigured, together with the at least one processor, the at least onecommunication module, and the at least one antenna, to cause the radioterminal (UE1) at least to determine (102) a SPS configuration for thefirst radio terminal (UE1), wherein the SPS configuration indicates aset (s1) of SPS, semi-persistently scheduled, radio resources (rr1-rr4)of a shared device-to-device radio channel (CH), wherein the indicatedSPS radio resources (rr1-rr4) are intended for a transmission by thefirst radio terminal (UE1); transmit (104), towards at least one secondradio terminal (UE2), data via a first subset (ss1) of the set (s1) ofthe SPS radio resources (rr1-rr4); select (106) at least one sensingresource (sr) out of the set (s1) of SPS radio resources (rr1-rr4);sense (108) the at least one sensing resource (sr); determine (110) acontention indicator (CONT) in dependence on the sensing (108) of the atleast one sensing resource (sr); and refrain (114) from transmittingdata via a second subset (ss2) of the set (s1) of SPS radio resources(rr1-rr4) if the contention indicator (CONT) indicates a contentionsituation at the at least one sensing resource (sr).
 2. The first radioterminal (UE1) according to claim 1, the first radio terminal (UE1)further configured to: transmit (112, 116), towards the at least onesecond radio terminal (UE2), data via the second subset (ss2) of the set(s1) of SPS radio resources (rr1-rr4) if the contention indicator (CONT)indicates no contention situation at the at least one sensing resource(sr).
 3. The first radio terminal (UE1) according to claim 1, the firstradio terminal (UE1) further configured to: determine (402; 502) atleast one auxiliary radio resource (ar1; ar2), wherein the at least oneauxiliary radio resource (ar1; ar2) and the set (s1) of SPS radioresources (rr1-rr4) are disjoint; and transmit (404, 406; 504, 506),towards the at least one second radio terminal (UE2), data via the atleast one auxiliary radio resource (ar1; ar2).
 4. The first radioterminal (UE1) according to claim 3, wherein the data transmitted viathe at least one auxiliary radio resource (ar1; ar2) comprises anout-of-sequence indicator (OOS), which indicates that the auxiliaryradio resource (ar1; ar2) is an auxiliary radio resource (ar1; ar2). 5.The first radio terminal (UE1) according to claim 1, wherein the atleast one sensing source (sr) is selected in dependence on a resourcereselection counter and a threshold for a probability of keeping the SPSradio resources.
 6. The first radio terminal (UE1) according to claim 1,wherein the selection (106) of the at least one sensing resource (sr)comprises selecting the at least one sensing resource (sr) out of theset (s1) of SPS resources (rr1-rr4) upon expiry of a time period (T1)since the first transmission via the set of SPS resources (rr1-rr4) orupon reaching a number of transmissions conducted via the set (s1) ofSPS resources (rr1-rr4).
 7. The first radio terminal (UE1) according toclaim 6, wherein the time period (T1) or the number of transmissions isdetermined in dependence on at least one selected from the groupconsisting of a measured channel load of the physical device-to-devicechannel, a present status of an egress queue of the first radio terminal(UE1), and a QoS indicator.
 8. The first radio terminal (UE1) accordingto claim 1, wherein determining (110) the contention indicator (CONT)comprises: decode SCI, Sidelink Control Information, present in thereceived at least one sensing resource (sr); determine a plurality ofradio resources (r2_UE2, r3_UE3) reserved for a further radio terminal(UE3); and determine the contention indicator (CONT) to represent nocontention, if the second subset (ss2) of the set (s1) of SPS radioresources (rr1-rr4) and the determined plurality of radio resources(r2_UE3, r3_UE3) are disjoint.
 9. The first radio terminal (UE1)according to claim 1, wherein the determining (110) the contentionindicator (CONT) comprises: determining a received signal strength forthe at least one sensing resource (sr); comparing the received signalstrength with a threshold; and determining the contention indicator(CONT) in dependence on the comparison.
 10. The first radio terminal(UE1) according to claim 1, wherein the sensing (108) comprises: sensinga first part of the sensing resource (sr); and transmitting, towards theat least one second radio terminal (UE2), data in a second part of thesensing resource (sr).
 11. The first radio terminal (UE1) according toclaim 1, wherein the refraining (114) comprises: determine (202; 412),after the SPS configuration has lapsed, a further SPS configuration forthe first radio terminal (UE1), wherein the further SPS configurationindicates a further set (fs) of further SPS radio resources (rr10, rr11)of the shared device-to-device radio channel (CH), wherein the indicatedfurther SPS radio resources (rr10, rr11) are intended for a transmissionby the first radio terminal (UE1); and start transmitting (206, 208;416, 418), towards the at least one second radio terminal (UE2), datavia the further set (fs) of SPS radio resources (rr10, rr11) if a timeperiod (T2) since the sensing (108) of the at least one sensing resource(sr) and before the first transmission via the further set of SPS radioresources (rr10, rr11) has not lapsed.
 12. The first radio terminal(UE1) according to claim 1, the first radio terminal (UE1) being furtherconfigured to: transmit (406), towards the at least one second radioterminal (UE2), the contention indicator (CONT), wherein the transmittedcontention indicator (CONT) indicates a contention situation at the atleast one sensing resource (sr) and the identity of the at least onesensing resource (sr).
 13. The first radio terminal (UE1) according toclaim 1, wherein the data transmitted towards the at least one secondradio terminal (UE2) comprises a reservation indicator, which indicatesan identity of the subsequent SPS radio resource (rr2; rr3; rr4) orsubsequent auxiliary radio resource (ar1; ar2) for transmitting by thefirst radio terminal (UE1).
 14. The first radio terminal (UE1) accordingto claim 1, wherein the data transmitted towards the at least one secondradio terminal (UE2) comprises a sensing indicator, which indicates anidentity of the subsequent SPS radio resource (sr) for sensing by thefirst radio terminal (UE1).
 15. A method to operate a first radioterminal (UE1) of a radio communications network (RCN), the methodcomprising: determining (102) a SPS configuration for the first radioterminal (UE1), wherein the SPS configuration indicates a set (s1) ofSPS, semi-persistently scheduled, radio resources (rr1-rr4) of a shareddevice-to-device radio channel (CH), wherein the indicated SPS radioresources (rr1-rr4) are intended for a transmission by the first radioterminal (UE1); transmitting (104), towards at least one second radioterminal (UE2), data via a first subset (ss1) of the set (s1) of the SPSradio resources (rr1-rr4); selecting (106) at least one sensing resource(sr) out of the set (s1) of SPS radio resources (rr1-rr4); sensing (108)the at least one sensing resource (sr); determining (110) a contentionindicator (CONT) in dependence on the sensing (108) of the at least onesensing resource (sr); and refraining (114) from transmitting data via asecond subset (ss2) of the set (s1) of SPS radio resources (rr1-rr4) ifthe contention indicator (CONT) indicates a contention situation at theat least one sensing resource (sr).
 16. A system configured to operate aradio communications network (RCN) which comprises the first radioterminal (UE1) according to claim 1 and the at least one second radioterminal (UE2).